Transmitters for use in communication terminals of communication systems include signal processing circuits such as a mixer for frequency conversion and an amplifier for power amplification. The transmitter processes an input modulation signal (baseband signal) using the signal processing circuits, and outputs the processed signal.
The operation of frequency conversion by a mixer as an example of the signal processing circuits will be described below.
If the mixer includes a DC (Direct Current) level shift in its characteristics or a baseband signal input to the mixer includes a DC component, then a high-frequency signal output from the mixer includes a carrier leak due to the DC offset. FIG. 1 is a graph showing the transmission spectrum of an ideal high-frequency signal, and FIG. 2 is a graph showing the transmission spectrum of a high-frequency signal including a carrier leak.
As can be understood from a comparison between FIGS. 1 and 2, the waveform (FIG. 2) of the signal including the carrier leak is different from the waveform (FIG. 1) of the ideal signal. The difference tends to degrade the EVM (Error Vector Magnitude) of the high-frequency output signal transmitted from the transmitter.
FIG. 3 is a block diagram showing the arrangement of a general transmitter. As shown in FIG. 3, the transmitter comprises signal generator 91, frequency converter 92, amplitude detector 93, and offset adjuster 94.
Signal generator 91 comprises a baseband circuit and generates and sends a baseband signal to frequency converter 92.
Frequency converter 92 comprises a mixer and frequency-converts the baseband signal input from signal generator 91 into an RF (Radio Frequency) signal, amplifies or attenuates the RF signal, and outputs the RF signal. The output from frequency converter 92 serves as the output of the transmitter.
Amplitude detector 93 comprises a spectrum analyzer and detects the amplitude of the RF signal output from frequency converter 92 and indicates the amplitude value thereof to offset adjuster 94.
Based on the amplitude value indicated by amplitude detector 93, offset adjuster 94 generates a DC offset correcting signal for correcting a DC offset, and feeds back the DC offset correcting signal to signal generator 91.
With the above arrangement, the transmitter shown in FIG. 3 monitors whether a carrier leak is found in the frequency spectrum of the high-frequency signal from the mixer or not. If a carrier leak is found, the transmitter adjusts the DC level to cancel the carrier leak using a circuit such as a DAC (Digital to Analog Converter) in signal generator 91 (baseband circuit), for thereby minimizing the DC offset component.
The relationship between the DC offset and the carrier leak in the RF transmission output will be described in detail below.
The mixer amplifies the power of the high-frequency output signal generated by mixing the baseband signal and a local signal, for thereby generating the output signal of the transmitter. Transmission output Pout of the transmitter which is in an ideal state free of a DC offset is expressed by equation (1):Pout=A(t)·sin(ωt)  (1)where A(t) represents the baseband signal input to the mixer and sin(ωt) the local signal.
The transmission spectrum of ideal transmission output Pout is shown in FIG. 1. In FIG. 1, the horizontal axis indicates the frequency and the vertical axis the intensity of signal component SGNL (generally referred to as a frequency spectrum) at each frequency. It is understood from FIG. 1 that signal component SGNL depending on the frequency can be obtained.
If baseband signal A(t) input to the mixer is an I/Q (In-phase/Quadrature-phase) signal and DC offset B is present in the I/Q signal, then transmission output Pout is expressed by equation (2):Pout=A(t)·sin(ωt)+B sin(ωt)  (2)
As can be seen from the equation (2), transmission output Pout contains carrier leak B sin(ωt) due to DC offset B. The frequency spectrum of transmission output Pout which contains the carrier leak is shown in FIG. 2.
In FIG. 2, the horizontal axis indicates the frequency and the vertical axis the intensity of signal component SGNL at each frequency.
As can be understood from FIG. 2, the frequency spectrum includes carrier leak CRLK due to DC offset B in addition to normal signal component SGNL.
It is known that the carrier leak caused by the DC offset component degrades the EVM of the high-frequency output signal of a communication system, thus degrading the communication quality.
It is also known with respect to communication systems that other circuits than the mixer, such as a baseband power amplifier that precedes the mixer, tend to cause a carrier leak due to the DC offset component. The carrier leak thus caused is also responsible for degrading the communication quality of the communication system.
It is preferable for the communication system to minimize the degradation of the communication quality caused by the DC offset component, and ideally to eliminate the DC offset component. It has been customary for transmitters according to the background art to detect the intensity of a carrier leak with a spectrum analyzer and adjust the DC level of the signal in a direction to minimize the intensity of the carrier leak.
As an arrangement for adjusting the DC level, there has been proposed a Cartesian loop negative-feedback amplifier for adjusting a DC offset that is input to an orthogonal modulator (see, for example, JP-A No. 10-136048).